1. Field of the Invention
The invention relates generally to a method for reducing power consumption in a mobile radio terminal such as a radio telephone, and more particularly, to a method for reducing power consumption in a standby mode of a radio terminal when the radio terminal is operated in a slotted paging mode in a CDMA (Code Division Multiple Access) radio telephone system.
2. Description of the Related Art
The slotted paging mode is a form of DRX (Discontinuous Reception) operations for a mobile radio terminal actuated by a battery such as a cellular radio telephone. The mobile radio terminal is adapted to radio communication with at least one remote base station in the radio telephone system. In the slotted paging mode, when the radio terminal (also referred to as a mobile station) is in an idle mode (or is not in conversation), the radio telephone is generally in a low power state without continuously monitoring a paging channel.
The slotted paging mode is important in the battery life of the radio telephone. It is an object of operating the slotted mode to reduce the operating time of a radio apparatus into the smallest amount and interrupt the power of the radio apparatus as little as possible during a sleep period. In an idle mode, the radio telephone wakes up only in a previously allocated slot by the radio telephone system or for treating certain other conditions such as a user input.
In returning from the sleep period, the radio apparatus should re-acquire an RF (Radio Frequency) link with the base station of the radio telephone system. Acquisition of the link and other operations including communication protocols for such a system are defined in air interface standards. An example of such a standard may include the TIA/EIA (Telecommunications Industry Association/Electronic Industry Association) IS-95: “Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System.” The IS-95 defines a DS (Direct Sequence)-CDMA or CDMA radio telephone system.
In order to re-acquire the RF link, the radio telephone of the CDMA system synchronizes with the system time that is maintained by the base station and a network controller of the CDMA system. The timing for a forward link (base station to mobile station) should be maintained by the radio telephone in anticipation that the radio apparatus is promptly started when an allocated slot is generated. The timing uncertainties are modified and the paging channel is acquired to prepare for treatment.
The synchronization with the forward link includes an alignment of a PN (Pseudorandom Noise) sequence transmitted to a pilot channel by the base station and a locally generated PN sequence. The transmitted sequences include a “short PN” sequence, which is repeated every 26⅔ ms, and a “long PN” sequence, which is repeated every 41 days. The radio telephone includes a sequence generator for generating short and long PN sequences same as those used by the base station. The radio telephone uses a searcher receiver or other mechanism for aligning the short PN sequence with that received from the base station. Once the pilot channel is acquired, the radio telephone acquires a synchronizing channel and the paging channel. Then, the radio telephone correctly demodulates a traffic channel to establish a full-duplex link with the base station.
In the start after the sleep time, the radio telephone synchronizes with the long PN sequence and the short PN sequence. The PN sequences and a frame boundary are repeated with a logical frequency in the IS-95 system. The frame boundary is generated in every third PN roll boundary. The PN roll boundary is defined by a short PN sequence which reverse rolls to the initial value thereof. In the mobile station, the short PN sequence and the long PN sequence are defined as a linear sequence. The short PN sequence and the long PN sequence are generated by using a LSG (Linear Sequence Generator). The LSG is described as a polynomial, and obtained by using a shift register and an exclusive OR gate. Since the short PN sequence is repeated only in every 26⅔ ms, the LSG, when withdrawing from the sleep state, can be conveniently stopped in a specific phase of the sequence until the phase has a correlation with a system PN. Then, the short PN LSG synchronizes with the system timing and starts again.
However, the long PN sequence is repeated only once in every 41 days. It is unpractical that the long PN generator of the radio telephone is stopped (for example, when entering the sleep state) and then the generator is clocked at high speed to catch up the long PN of the system in the start thereafter.
Since the short PN sequence and the long PN sequence are transmitted by the system which varies predictably according to time, the PN sequence acquisition requires that a correct reference time is maintained in the mobile station during the sleep mode. A PN sequence can be suitably determined for the correlation with the system PN sequence in withdrawing from the sleep mode. However, maintaining a precise timing reference requires a relatively high power consumption which is contradictory to the sleep mode designed for low power consumption.
In addition to withdrawing from the sleep mode during the allocated slot, the radio telephone is also required to start for handling other asynchronously generated events in the radio apparatus or for responding to the same. An example of this type of event can include a user input such as pressing a keypad of the radio telephone. The response to such an input should be prompt without any delay which can be recognized by the user.
As it is important in the foregoing related art, a technique for reducing power consumption uses a method in which a receiver in a standby mode is activated only at a pre-designated time to perform a receiving operation during a certain time and then returns to the low power mode by using the slotted paging mode supported by the system. Here, even in the low power mode, minimum clocking is required for the purpose of maintaining synchronization with the system being re-activated and responding to the external input such as an interrupt.
However, while an edge timing should be suitably used in order to use the low frequency clock, actually a mode operated in a high frequency clock is present between receiving modes to maintain the correct edge timing required by the system. Such a mode is referred to as catnap mode in the following description. The operation of this catnap mode is defined with a certain period, and this period is defined via a trade-off according to an experiment.
However, if the operating period of the catnap mode is shorter than required, a loss is caused in power consumption, and if too long, the system may not be suitably synchronized in some cases.